1. Field of the Invention
The present invention relates to a superlattice device and a manufacturing method thereof, and more particularly relates to a superlattice device having a crystal layer of which crystal structure can reversibly change by application of energy, and relates to a manufacturing method of the superlattice device. The present invention also relates to a solid-state memory including a superlattice device, a data processing system, and a data processing device.
2. Description of Related Art
At present, a so-called phase-change material is widely used as a material of a recording layer of rewritable optical disks such as DVD-RW. A chalcogen compound containing antimony (Sb) and tellurium (Te) as main components is known as a representative phase-change material. Information is recorded by using a difference between an optical reflectance in a crystalline phase and an optical reflectance in an amorphous phase. A crystalline phase of the chalcogen compound is changed to an amorphous phase by heating the chalcogen compound at a temperature equal to or higher than a melting point by irradiating a laser beam, and thereafter by rapidly cooling this chalcogen compound. On the other hand, an amorphous phase of the chalcogen compound is changed to a crystalline phase by heating the chalcogen compound at or higher than a crystallization temperature and lower than a melting point, and thereafter by gradually cooling the chalcogen compound.
In recent years, the phase-change material is commanding attention as a material of a recording layer of a semiconductor memory as well as that of a recording layer of an optical disk. When the phase-change material is used as a material of a recording layer of a semiconductor memory, information is recorded by using a difference between an electric resistance in a crystalline phase and an electric resistance in an amorphous phase. The semiconductor memory of this kind is generally called PRAM (Phase change Random Access Memory).
Conventionally, a bulk-shaped phase-change material is used for this kind of device. However, some of present inventors have found out that an energy necessary for a phase change is substantially decreased by forming a superlattice using two kinds of phase-change materials having mutually different compositions (see International Publication Nos. WO2009/028249 and WO2009/028250).
However, it is not easy to form a superlattice with a phase-change material, and its device characteristic greatly changes depending on a process condition. As a result of detailed examinations to solve this problem, the present inventors have found out that a surface state of an underlaying layer of the superlattice laminate gives a large influence on the device characteristic.
For example, International Publication No. WO2009/028250 describes a superlattice structure having a GeTe layer and a Sb2Te3 layer laminated alternately. While the GeTe layer is a distorted NaCl-type cubic crystal, the Sb2Te3 layer is a hexagonal crystal. Therefore, a laminated surface of the GeTe layer needs to be (111)-orientated to obtain a superlattice structure from the GeTe layer and the Sb2Te3 layer. However, because a crystal direction of the GeTe layer strongly depends on a surface state of an underlaying layer, it has become clear that there are many lattice disturbances in the superlattice structure depending on the surface state of the underlaying layer. The present invention has been achieved based on such technical findings.